Update lightning requirement (#648)

Updates the requirements on [lightning](https://github.com/Lightning-AI/lightning) to permit the latest version.
- [Release notes](https://github.com/Lightning-AI/lightning/releases)
- [Commits](https://github.com/Lightning-AI/lightning/commits)

---
updated-dependencies:
- dependency-name: lightning
  dependency-version: 2.6.0.dev20250921
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

pyproject.toml

@@ -58,7 +58,7 @@ dependencies = [
     "torch<=2.4.0; sys.platform == 'linux'",
     "torchdata<=0.8.0; sys.platform == 'linux'",
     "pymatgen",
-    "lightning<=2.6.0.dev20250914",
+    "lightning<=2.6.0.dev20250921",
     "pydantic",
     "boto3",
     "numpy<2.0",